1. Field of the Invention
The invention relates to fluorescing compositions which cure by exposure to ultraviolet radiation and contain a compound which fluoresces at visible wavelengths. More particularly, the invention pertains to liquid, radiation-curable compositions comprising a substantially homogeneous fluid comprising a fluorescing europium compound in an admixture of a polymerizable component and a polymerization initiator. Such radiation-curable compositions find use as adhesives, coatings, and the like. In particular, the integrity of the cured composition can be inspected since they fluoresce at visible wavelengths.
2. Description of the Related Art
It is known in the art to produce light sensitive adhesive compositions which adhere to critical surfaces. For examples, in the field of medical devices, it is important to inspect catheter interfaces to assure that adjacent surfaces are actually bonded. It is known in the art to incorporate fluorescent compounds into coating compositions to provide a method for inspection. Methods are also known for detecting uniformity and identifying defects in coatings that have been applied to substrates. The prior art describes the incorporation of fluorescing agents into UV-cured coatings as a means for identifying the presence of the cured film and for ensuring that they are properly covered with the coatings. The use of UV coatings in such applications has created a requirement for measuring and monitoring the quality and depth of the coating.
The curing of radiation curable compositions depends upon the correct wavelength of light striking photoinitiators to generate free radicals by which polymerization of polymerizable components is effected and thereby to form a desired polymeric film. In the art, many of the fluorescing agents that are used for inspecting the quality of cure, such as substituted oxazole compounds and fluoranthenes, absorb radiation in the same region of the spectrum as the photoinitiators employed to generate the required free radicals. The resultant filtering or blocking phenomenon has limited the concentration of fluorescing agent that can be incorporated into a coating or adhesive formulation, since an excessive amount of the agent will preclude adequate reaction of the ingredients and adequate depth of cure. This has impeded the acceptance of UV compositions for coatings that require a bright fluorescent response. Small amounts of a fluorescing agent can generally be incorporated into compositions containing conventional UV-curing photoinitiators without significant detriment to the depth of cure achieved. However, when the level of fluorescent material is increased to improve the brightness of response, it is often found that the coating will not cure properly. Exposure to radiation in both the ultraviolet and also the visible spectral regions can have the additional effect of decomposing the fluorescent agent molecule, thereby further diminishing the response of the coating to light. The use of photoinitiators which respond to the visible part of the spectrum is one method to diminish the filtering effect of the fluorescing agent. However, these impart a red or dark yellow color to the resulting film or adhesive, and hence are undesirable. The prior art describes the use of mono and bisphosphine oxides as photoinitiators, which can provide excellent depth of cure in UV-curable coatings. The success of these phosphine oxides is attributed to their ability to respond in the near-UV/visible spectral region and to photo-bleach. One solution proposed by U.S. Pat. No. 6,080,450 uses a combination of a visible absorbing fluorescent agent with a phosphine oxide photoinitiator. The formulations contain less than 0.5% phosphine oxide. The phosphine oxide absorbs up to 440 nm, with a peak at 380 nm. The depth of cure is good. However, the amount of phosphine oxide must be increased to maintain the same depth of cure when the amount of fluorescing compound is increased. One commercial product uses diphenyl(2,4,6-trimethylbenzoyl)phosphine oxide (CAS 75980-60-8) in a blue fluorescing adhesive composition. Due to its lower absorption in the visible light region, more phosphine oxide is typically required. The phosphine oxides cause the liquid formulation to be slightly yellow. Typical ketone photoinitiators absorb light in the ultraviolet region, not in the visible region. Therefore, they do not discolor the formulations. However, these ketone photoinitiators are unsuitable in combination with blue fluorescing compounds that absorb in the visible light region. Other inorganic fluorescing compounds, such as zinc salts do not dissolve or disperse well in typical monomers. Organic non-blue fluorescing compounds, such as benzoxazinone derivatives also have a broad emission spectrum, however, they do not fluoresce after being dissolved in typical polymerizable monomers. Other organic fluorescing compounds with reactive side groups only fluoresce weakly after curing.
It has now been found that fluorescing compounds that do not absorb in the visible light region do not require a phosphine oxide as the photoinitiator. In particular, it has been found that rare earth metal compounds, such as europium compounds, are suitable. These fluorescing compounds have certain advantages, such as monochromatic red emission detectable in blue surroundings; easy solubility in polymerizable monomers; and non-leaching in the cured state. Europium chelates are well known in the prior art as diketonate and carboxylic acid complexes. The carboxylic acid complexes tend to be more light and heat stable. The europium chelates are readily soluble in typical polymerizable monomers.